Scientists to convene at world’s largest cosmic-ray observatory Nov. 10-11

Collaborating scientists from 16 countries will rendezvous in Malargüe, Argentina, Nov. 10 and 11 for a celebration at the Pierre Auger Observatory, the largest cosmic-ray detector ever built.

On the schedule of events is a symposium with presentations on the origins of the project, its construction and its first science results; tours; and a science fair featuring participants from local schools.

Auger scientists seek to pierce the secrets of a phenomenon that has baffled scientists for nearly seven decades: what is the mysterious source from beyond the galaxy that periodically bombards the Earth with high-energy cosmic rays?

“These particles might be coming from jets of matter emitted by supermassive black holes. Or the particles could be accelerated by the magnetic fields of fast-spinning neutron stars. It could even be some effect coming from extra dimensions,” said the University of Chicago’s Angela Olinto, an Auger collaborator.

High-energy cosmic rays consist of protons and other subatomic scraps of matter that fly through the universe at nearly the speed of light. The most powerful cosmic rays contain more than a hundred million times more energy than the particles produced in the world’s most powerful particle accelerator. When these rays collide with air molecules in Earth’s atmosphere, they trigger a shower that multiplies into billions of secondary particles before they reach the ground.

University of Chicago Nobel laureate James Cronin initiated the Auger project with Alan Watson of the University of Leeds in the early 1990s. They named the project for Pierre Auger, the French scientist who in 1938 discovered the existence of these particle showers.

“This is a new form of astronomy,” said Cronin, University Professor Emeritus in Physics at the University of Chicago. “The basic thrust for this is coming from particle physics.”

Construction of the observatory began in March 1999. More than half the observatory’s 1,600 detectors are now collecting data. When completed late next year, the observatory will consist of a grid of cosmic-ray detectors and associated electronic instruments that covers 1,200 square miles of the vast plain known as the Pampa Amarilla in western Argentina, approximately 600 miles west of Buenos Aires.

The Auger collaboration established the observatory in Argentina in part because earlier cosmic-ray experiments were set up in the Northern Hemisphere. “The Southern Hemisphere really has not been properly explored,” Cronin said.

The $50 million observatory must cover a wide area because high-energy cosmic rays are rare. Only one of these particles strikes any given square mile of the Earth in a century. To detect these particles, the observatory combines two techniques that had previously been employed separately by earlier cosmic-ray observatories built in Utah and Japan.

One technique consists largely of simple plastic water tanks, which serve as ground detectors. Measuring five feet tall and 12 feet in diameter and spaced at one-mile intervals, they will occasionally intercept a particle from the atmospheric cascade generated by cosmic rays. As the particle crosses from air into water its speed changes, producing a supersonic shock, must like an airplane does when it crosses the sound barrier, explained Olinto, an Associate Professor and Chair of Astronomy & Astrophysics at the University of Chicago.

“The shock that it creates—instead of being a boom like you hear in airplanes—is a flash of light. That light is then detected in this dark chamber in the water, and we have that information radioed to a central station,” Olinto said.

The second technique involves fluorescence telescopes that can detect the ultraviolet light emissions generated in the atmosphere by cosmic rays. “With these two techniques we can actually see the shower developing in the atmosphere and see the leftovers of the shower as they hit the ground,” Olinto said.

Previous experiments concentrated on just one technique or the other and on a smaller scale. The High Resolution Fly’s Eye experiment in Utah detects cosmic rays by observing the fluorescent light they cause when they strike the atmosphere. Japan’s Akeno Giant Air Shower Array (AGASA) detects the cascade of secondary particles when they strike the ground.

Those two experiments have yielded conflicting results. AGASA data shows a hint of particle clusters coming from the same region of space, along with cosmic rays at unexpectedly high energies. The Fly’s Eye shows neither. The early Auger data also show discrepancies between the two techniques. The discrepancies could be the result of a calibration problem with the detectors, or an entirely new phenomenon in physics.

“Really understanding the origin of these particles should open up a lot of different questions that we would like to answer next,” Olinto said. “But we have to start by knowing where they come from.”